hide Metadata assert insides opendal's own tests features,

[Xuanwo]

          OpenDAL has an assertion that to prevent users to access metadata that not been set. For example, visting etag from the metadata returned by list.

I feel the assertion from opendal is too aggressive. Perhaps we could leave those fields as None if they're not set.

We can hide those assert insides opendal's own tests features, so they won't be exposed to user side.

opendal/core/src/types/metadata.rs

Lines 140 to 144 in 9d587ea

	debug_assert!(
	self.metakey.contains(Metakey::CacheControl)
	|| self.metakey.contains(Metakey::Complete),
	"visiting not set metadata: cache_control, maybe a bug"
	);

Originally posted by @Xuanwo in #4581 (comment)